Real-Time & Sampling Oscilloscopes (Continued)

Another oscilloscope technology is known as equivalent time sampling. These sampling oscilloscopes have a base digitizer that is considerably slower than a real-time oscilloscope’s digitizer, but their input bandwidth can exceed 70 GHz, which is possible because sampling oscilloscopes don’t use a repeated signal and phase-coherent trigger. (Phase coherent signifies that the trigger signal happens at the same point in the waveform every time.) In addition, the trigger path is a separate connector, meaning you have to either split a repeating signal to create the signal or use an external clock source to supply the trigger. Although the majority of contemporary sampling oscilloscopes use sequential sampling, each trigger still yields just one sample.

So why would you want an equivalent-time sampler? First, you won’t find a 70 GHz real-time oscilloscope, and a 20 GHz sampling oscilloscope costs much less than a 20 GHz real-time oscilloscope. What’s more, an equivalent-time oscilloscope will have a faster sampling resolution, which is to say femtoseconds rather than picoseconds. Equivalent-time instruments have additional bits which improves the vertical resolution. Lastly, sampling oscilloscopes often have twelve- or fourteen-bit digitizers that enable you to analyze noise better and improve jitter measurement, which isn’t possible with an 8-bit digitizer.

Sampling oscilloscopes require an understanding of your signal and what you would like to measure, which is to say you can’t go poking around the circuit and expect to stumble on a solution to your problems.

There aren’t nearly as many sampling oscilloscope vendors as there are real-time oscilloscope makers because sampling oscilloscopes tend to be a part of mainframe systems in which you purchase the mainframe and pick and choose plug-in modules to suit your needs.

Note that a large number of real-time oscilloscopes are capable of equivalent-time sampling as long as you’re able to trigger your signal; however, you do not need a separate trigger path added to the front.

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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